The use of a variety of light redirecting films in backlit displays is well known. Light redirecting films are typically thin transparent optical films or substrates that redirect the light passing through the films such that the distribution of the light exiting the films is directed more normal to the surface of the films. The output light intensity normal to the film divided by input light intensity normal to the film is called the “on-axis gain” of the film. Typically, light redirecting films are provided with ordered prismatic grooves, lenticular grooves, or pyramids on the light exit surface of the films which change the angle of the film/air interface for light rays exiting the films and cause the components of the incident light distribution traveling in a plane perpendicular to the refracting surfaces of the grooves to be redistributed in a direction more normal to the surface of the films. Such light redirecting films are used, for example, to increase brightness in liquid crystal displays (LCDs) in laptop computers, televisions, desktop monitors, cell phones and the like.
Previous light redirecting films suffer from visible Moiré patterns when the light redirecting film is used with a liquid crystal or other display. The surface elements of the light redirecting film interact with other optical films, the pattern of printed dots or three-dimensional elements on the back of the light guide plate, or the pixel pattern inside the liquid crystal modulator to create Moiré, an undesirable effect. Methods known in the art for reducing Moiré include die cutting the light redirecting films at an angle to change the average pitch of the linear array, randomizing the linear array by widths of the linear array elements, varying the height along the linear array, or adding diffusing films to the display assembly. The above techniques to reduce Moiré also cause a decrease in on-axis brightness or do not work to adequately solve the Moiré problem. Moiré and on-axis brightness tend to be related, meaning that a film with high on-axis gain would have high Moiré in a system. It would be beneficial to be able to reduce the Moiré while maintaining sufficient on-axis gain.
Previous light redirecting films also suffer from high quality requirements and production costs. Very small defects in linear arrays of prisms, stray contamination particles, and small scratches can be visible in the film and assembled display. In addition, defects on lower layers of the display assembly are often visible through the light redirecting film. As a result the films suffer from high reject rates and low yield, or they must be manufactured to exacting standards, in clean rooms, and with great care in handling during fabrication and assembly into displays. Methods known in the art for making the films more resistant to defect visibility include varying the height of the prisms along the linear array, as disclosed in U.S. Pat. No. 6,354,709 (Campbell et al.), or adding diffusing films to the display assembly. The above techniques increase cost, cause a decrease in on-axis brightness, or do not adequately hide defects. It would be beneficial to be able to hide defects in and under the light redirecting film while maintaining high on-axis gain.
U.S. Pat. No. 5,919,551 (Cobb, et al) claims a linear array film with variable pitch peaks and/or grooves to reduce the visibility of Moiré interference patterns. The pitch variations can be over groups of adjacent peaks and/or valleys or between adjacent pairs of peaks and/or valleys. While this varying of the pitch of the linear array elements does reduce Moiré, the linear elements of the film still interact with the dot pattern on the backlight light guide and the electronics inside the liquid crystal section of the display. It would be desirable to break up the linear array of elements to reduce or eliminate this interaction.
U.S. Pat. No. 6,752,505 (Parker et al.), incorporated herein by reference, discloses the use of individual optical elements for redirecting light, including individual optical elements of varying size and shape. However, light redirecting films with varying individual optical elements can have unexpected problems that can be difficult to solve, including loss of gain, varying gain, cosmetic defects, and visually objectionable patterns. It would be desirable to have a film that achieves the advantages of varying individual optical elements yet avoids these problems.